With increasing public awareness of environmental issues, there are concerns about the safety of chemical substances. In line with this trend, technical development has been promoted to replace organic solvents with carbon dioxide and like alternative solvents posing less environmental burden. However, due to the properties of carbon dioxide such as low polarity, the miscibility therewith of polar substances such as water, polymers, contaminants is significantly poor. To solve the miscibility problem, the use of CO2-philic surfactants has been proposed, and the development of surfactants and novel systems that use such surfactants have been actively carried out. Examples of known surfactants are polyoxyalkylenes, polysiloxane, and fluoroorganic compounds. Among such surfactants, fluorine compounds are functionally superior. However, fluoro compounds are generally expensive and problematic in disposal, posing more severe environmental burdens than the two other types of compounds. Therefore, developing a method for reusing fluoro compounds is a great challenge. Although development of fluoro compounds and systems using such fluoro compounds in combination with supercritical carbon dioxide is currently promoted, no effective technique for the recovery of the surfactant has been established. To date, the only method known is decreasing the solubility of such a surfactant through pressure control to precipitate and then recover it (J. Am. Chem. Soc., 1999, 121, 11902; Chem. Eng., 2000, 72).
Indeed, prior-art surfactant separation and recovery methods are for removing surfactants that are problematic to waste water treatment from surfactant-containing aqueous solutions. Examples of such prior-art techniques are given below. However, there is no method at present that allows the recovery from a reverse micelle emulsion, in which water is dispersed in carbon dioxide, of just a surfactant with a purity suitable for reuse as desired by the inventors. Moreover, there is no technique that relates to a system allowing the efficient recovery and reuse of a surfactant in a high-pressure system of liquid, subcritical or supercritical carbon dioxide.
Prior-art methods for recovering surfactants are presented below:
1. Precipitation of a Surfactant By Salting Out or Addition of an Adsorbent or Flocculant
This method requires surfactant purification by removing a salt or flocculant (Japanese Patent No. 2881384, Japanese Unexamined Patent Publication No. 2000-093944).
2. Removal of Water By Evaporation
This method uses a large amount of energy, takes a long period of time, and does not enable a surfactant to be recovered with a high purity. Moreover, this method is not usable with supercritical CO2 equipment (Japanese Unexamined Patent Publication Nos. 1993-140779 and 1996-164301).
3. Centrifugation
A surfactant after centrifugation that contains impurities has to be purified using a method other than centrifugation. Moreover, water cannot be completely removed. This method takes a long period of time and requires a centrifuge. This method is not usable with supercritical CO2 equipment (Japanese Patent No. 3382462).
4. Separation By a Separation Membrane (Ultrafiltration Membrane, Reverse Osmotic Membrane, Ion-Exchange Membrane)
This method requires expensive membrane separation equipment. Water cannot be completely removed with either ultrafiltration membranes or reverse osmotic membranes (a surfactant is obtainable only in the form of a concentrated aqueous solution). Ion-exchange membranes are not suitable for nonionic surfactants. With ionic surfactants, a process for extracting a surfactant adsorbed onto the membrane is necessary. Furthermore, the durability of organic membranes in supercritical CO2 is questionable (Japanese Unexamined Patent Publication Nos. 1998-230149, 2002-058966, 1993-317654, 2002-059160, and 1996-020611).
5. Precipitation of a Surfactant By Cooling
This method needs energy for cooling and takes a long period of time. Impurities also precipitate as solids, necessitating a process for purifying the surfactant (Japanese Unexamined Patent Publication Nos. 2000-210679 and 1996-155205).
6. Separation Into Water and an Oil Component Containing a Surfactant by Applying an Electric Field in Order to Neutralize the Electric Charge of the Surfactant.
It is difficult to use this method for nonionic surfactants, and an electrolytic device is needed with this method and the procedure is complex and time-consuming. It is difficult to use this method with supercritical CO2 equipment (Japanese Patent No. 3358786 and Japanese Unexamined Patent Publication No. 1997-164303).
7. Separation by Increasing Temperature Above the Cloud Point to Lower the Water Solubility of a Surfactant
This method takes a long period of time and necessitates separation from impurities. Moreover, it is impossible to completely remove water (Japanese Unexamined Patent Publication Nos. 1999-033505, 1994-063534, 1993-269456, 1996-259471, 1996-332305 and 2003-088879).
With the drive toward smaller feature sizes of resist patterns, when resist patterns are washed with water and dried, the problem of image collapse, which occurs due to the capillary force of water, has been focused. In particular, when the pattern width of a semiconductor device is about 80 nm, pattern collapse is likely to occur, and 65 nm or finer patterns are even more likely to suffer pattern collapse.
In order to avoid pattern collapse, a method in which supercritical carbon dioxide is used has been proposed as a method for drying fine patterns. However, since resists are made from polymers, when they are treated with high-pressure carbon dioxide, they undergo bubbling and deformation due to water absorption. It is now understood that to prevent such phenomena, drying has to be performed after removing water with liquid carbon dioxide (Namatsu et al., Chorinkairyuutaino saishin ouyou gijyutsu (Latest application techniques for supercritical fluid), NTS, pp. 307–324; Otake et al., Surface, 2004, Vol. 40, No. 10, p. 360; J. Vac. Sci. Technol. B18(2), 780–784 (2000); Proceedings of SPIE, Vol. 5037 (2003). Heretofore, there has been no surfactant that can solubilize water in a pressure range in which carbon dioxide is liquid. Although the application of supercritical carbon dioxide to semiconductor processes other than resist pattern drying has been researched, even state-of-the-art high performance surfactants necessitate a pressure of about 300 atm to dissolve water in carbon dioxide in a proportion of several percent. Moreover, there are many problems with the recovery process, and it is also a problem that expensive surfactants have to be used in large amounts. As described above, it has been extremely difficult to handle polar substances and polymers in carbon dioxide with the use of small amounts of a cosolvent at a pressure of 100 to 150 atm, or lower practical pressures.
An object of the present invention is to provide a technique for reusing a surfactant without separating it from carbon dioxide in a water-carbon dioxide system containing the surfactant.
Another object of the present invention is to provide a method for washing, drying, or dehydrating precision machinery components or semiconductor wafers having fine patterns without resulting in pattern collapse.
Furthermore, another object of the present invention is to provide a technique for selectively removing a removal target in a mixed system containing carbon dioxide, a carbon dioxide-compatible surfactant and/or a cosolvent, and a removal target.